The invention relates to a magnetic field sensor apparatus for measuring at least one magnetic field vector component comprising at least one anisotropic magneto-resistive resistor device arranged on a chip substrate. The resistor device comprises a plurality of magneto-resistive AMR resistor elements which are contacted in series by electrically conductive strips.
Electronic magnetic field sensor apparatus based on the mode of operation of AMR resistor layers are well-known from the prior art. The AMR effect (anisotropic magneto-resistive effect) is the property of a material to change its electrical resistance depending on an angle between an electrical current flowing through the material and an external magnetic field according to the position of the magnetic field vector relative to the flow of current through the material. This effect can be observed in particular in very long, thin and narrow strips of permalloy. Permalloy is an alloy consisting of 81% nickel and 19% iron. The electrical resistance of this material is greatest when the electric current is flowing in the direction of a magnetic field vector component. It is at its lowest when the electric current is aligned perpendicular to the electrical magnetic field vector component. On the molecular level, this effect is caused by a disturbance in the alignment of the electron spin of the atoms that depends on the direction of the electrical magnetization. In general, in the forementioned configuration, the electrical resistance is given by R=R⊥+(R=R⊥) cos2Θ, where θ is the angle between the magnetic field vector component and the direction of the flow of current in the AMR measuring strip. As can be seen in the above equation, a high sensitivity, i.e. a big change in resistance as the magnetic field to be measured changes, is particularly great when the current flows at about 45° to the magnetic field vector component. With this angular relationship between the current and the vector component, the change in resistance relative to a change in the magnetic field becomes linearized. The relationship between the measurement current IS and a magnetic field M(H) causing a change in the resistance, expressed above as a formula, is shown in FIG. 1. The magnetic field M(H) causing a change in the resistance is here symbolized by an initial magnetization M0 in the direction of the flow of current IS together with a vector component He perpendicular thereto provided by the external magnetic field to be measured.
The arrangement of what are known as barber's pole structures, i.e. thin, conductive structures arranged on an AMR measuring strip and consisting of extremely conductive material such as, for example, aluminum, copper, gold or silver, is known from the prior art for the design of magnetic field sensor apparatus. The barber's pole structures are aligned at 45° relative to the longitudinal extent of the AMR resistor strip, and are reminiscent of the advertising sign for a barber's shop in the USA, which is why they are called barber's pole structures. A current flowing through the resistor strips is thereby forced to flow in a direction at 45° to the longitudinal extent of the strip, as is shown in FIG. 2. As a result, the curve showing the dependency of the resistance on the alignment of the current vector shown in FIG. 1 is shifted by 45° relative to the magnetic field vector, so that it can, as shown in FIG. 2, be shifted to a linearized region. Depending on the alignment of the barber's pole structures, a positive or negative edge of a linearization between the resistor and the magnitude of the magnetic field to be measured is the result, provided that the external magnetic field is aligned longitudinally or transversely to the extent of the AMR resistor strip.
Magnetic field sensor apparatus based on barber's pole structures are known from, for example, DE3442278A1. Four magnetic field sensor apparatuses of this type with differently aligned barber's pole structures are here connected together in a Wheatstone measuring bridge, where an initial magnetization of the internal magnetization M0 of the resistor strips is achieved by an external magnetic field generated by a macroscopic magnetic coil, in order to create a linearized dependency of the resistance on the external magnetic field He. For this purpose, a magnetic field is generated perpendicular or parallel to the current flow, generating an initial magnetization of the AMR resistor measuring strips, so that a magnetic field He to be measured and aligned perpendicular creates a linear change in the electrical resistance of the magnetic field sensor apparatus, which is measurable and from which the magnitude of the external magnetic field He can be determined.
An improvement to a design of this type is described in, for example, DE4319146C2, which proposes that a series of AMR resistor devices with barber's pole structures is arranged along a conductor through which a flip current is flowing, with the flip current conductor effecting an initial magnetization M0 in the longitudinal direction of the AMR measuring strips. By flipping, i.e. reversing of the internal magnetization, a realignment or calibration of the resistance behavior can be achieved. An external magnetic field He perpendicular thereto can be measured with high sensitivity. In order to generate an initial magnetization M0 in the longitudinal direction of the AMR resistor strips, a flip current If flowing through a flip conductor is used, as a result of which an increased current has to be used to provide an initial magnetization. The application of the barber's pole structures is disadvantageous from the point of view of process technology, as very fine microstructures have to be arranged on the AMR resistor strips, and short circuits between the individual strips have to be avoided. The arrangement of the flip conductor increases the number of manufacturing steps, and the control of the flip current increases the demands made on the control electronics.
The ratio of R⊥ to R− is in the region of 3% for permalloy. The result of the anisotropic shape is that magneto-resistive resistors, in which the strips are particularly thin and the length is chosen to be large in comparison with the width, exhibit a preferred magnetization direction as a result of the anisotropic shape in the longitudinal direction, i.e. parallel to the direction of the current. In order to impress an initial magnetization, and to stabilize a magnetization in a resistor strip, the provision of additional macroscopic permanent magnets mounted in the vicinity of the magneto-resistive layer structure is known. These permanent magnets are intended to prevent the internal magnetization of the strips from reversing. Additional permanent magnets of this sort are described for example in DE4221385C2. A particular disadvantage of this solution is the high assembly effort, along with the resulting structural size and the costs of the components used.
EP0766830B describes a magneto-resistive reading head for a digital magnetic storage medium. EP0585008A2 presents a further magneto-resistive reading head for a digital magnetic storage medium, e.g. for a hard disc application, having two permanent magnets between which a single magneto-resistive resistor element is arranged. The two permanent magnet elements provide an internal magnetization direction inside the resistor element. The resistor element is contacted by electrically conductive strips in such a way that a measurement current, displaced about 45° relative to the magnetization direction, can flow through the resistor element. This configuration is selected so that a magnetization location carrying bit information of a ferromagnetic magnetic storage medium, e.g. of a magnetic tape or of a magnetic storage disk, gives rise to a defined rectangular and symmetrical change in resistance, with steep edges and minor spatial/temporal displacement in the resistor element, so that a magnetic bit storage density is increased, crosstalk between adjacent magnetic field bits is suppressed, and analog server-data can be read out symmetrically. The reading head is optimized to detect at high speed binary magnetic field information with a similar spatial extent to the resistor element itself, and is not suitable for detecting a strength and direction of a magnetic field component of an external magnetic field, e.g. for a compass or current measuring application.
For this reason, the previously known AMR magnetic field sensor apparatuses result in the problem that using barber's pole stripes for linearization of the characteristic resistance curve means that on the one hand the process technology must arrange very fine barber's pole structures on the resistor strips, and on the other hand external circuitry and an additional flip conductor or external magnets for initial magnetization or for maintaining the internal magnetization must be provided, which raise the current consumption and increase the complexity of the electronic circuit.
U.S. Pat. No. 6,822,443 B1 demonstrates a magnetic field sensor apparatus for the measurement of two magnetic field components at an angle to one another. The magnetic field sensor apparatus comprises two Wheatstone measuring bridges that can be evaluated separately from one another. Each measuring bridge comprises two magnetically neutral resistors and two magneto-resistive resistors. The magneto-resistive resistors can be AMR/SDT/GMR or Hall resistors. In order to increase the sensitivity to magnetic fields, flux concentration elements can be arranged in the region of the magneto-resistive resistors, and consist of NiFe, are unmagnetized and referred to as a “free ferromagnetic layer”.
WO 2012/103950 A1 presents an AMR resistor device whose bridge resistors consist of a series connection of individual resistor elements, where the arrangement of the resistor elements of one bridge resistor meshes interdigitally with the arrangement of the resistor elements of a second bridge resistor.
It is desirable to propose an AMR magnetic field sensor apparatus in which a highly sensitive linearized characteristic resistance curve can be achieved without having to accept the disadvantages of external macroscopic support magnets, barber's pole structures and a flip current conductor, so that an economical magnetic field sensor apparatus can be obtained.
In accordance with an aspect of the invention, the magnetic field sensor apparatus for measuring a magnetic field vector component He comprises one anisotropic magneto-resistive device (AMR resistor device) arranged on a chip substrate, where the resistor device comprises a plurality of magneto-resistive AMR resistor elements which are contacted by electrically conductive strips. At least one permanent-magnetic magnetization element with a magnetization axis is assigned to each resistor element in such a way that the resistor element is passed through by an initial magnetization field H0 of the magnetization element in the direction of the magnetization axis. The invention is based, according to an aspect thereof, on the fact that a measurement current IS, flowing through the resistor element from a contact region between a first conductive strip and the resistor element to a contact region between the resistor element and a second conductive strip, has a mean current direction axis at a predefined linearization angle α>0° and α<90° relative to the magnetization axis.
In other words, a magnetic field sensor apparatus is proposed comprising at least one and in particular a plurality of resistor devices, i.e. separately connectable AMR resistors, where each resistor device comprises a plurality of AMR resistor elements connected electrically in series. The resistor elements are contacted by electrically conductive strips in such a way that current can flow in series through the AMR resistor elements, where one or two magnetization elements whose initial magnetization field H0 results in an internal initial magnetization M0 inside the resistor element are assigned to each resistor element. The arrangement of the magnetization element relative to the direction of flow of the current, which is predefined by the alignment and shape of the conductive strips and their contact to the resistor element, is selected in such a way that a linearization angle α is created between the magnetization axis of the magnetization element and the direction in which the measurement current is flowing, and can be adjusted between 0° and 90°. As a result, as is shown comparatively in the characteristic resistance curves in FIG. 2, a displacement of the characteristic resistance curve can be adjusted in the direction of a linear region α=45° between the magnetization vector and the direction of current flow, so that a linearized characteristic curve of the resistance depending on an external magnetic field He can be achieved. The current through the resistor element can flow at an angle relative to the current through the conductive elements, where the magnetization vector can be aligned parallel or perpendicular to the direction of the flow of current through the conductive elements. Alternatively, the directions of the flow of current through the conductive elements and the resistor element can be identical, whereas the magnetization vector is aligned at an angle α relative to the overall direction of the flow of current.
It is for example possible for a single magnetization element to provide an initial magnetization M0 for a plurality of resistor elements, although preferably a separate magnetization element or a pair of magnetization elements oriented in the same direction can be assigned to each resistor element. The direction of the flow of current within the resistor element can be set by the position of the contact of the resistor element with the conductive strips—i.e. the shape of the AMR resistor elements and the position of the contact locations between the conductive strips and the resistor element. This can preferably be adjusted to be neither parallel nor perpendicular to the alignment of the series of AMR resistor elements. The position of a magnetization element with magnetization in the direction of the longitudinal or transverse alignment of the resistor elements defines, relative to the direction of current flow through the resistor element, the linearization angle α. Due to the structural arrangement of the magnetization elements relative to the direction of the flow of current through the AMR resistor elements, a displacement of the characteristic resistance curve can be achieved in such a way that a linearization of the resistance is provided dependent on the magnetic field. The arrangement of barber's pole strips and the artificial creation of a flip magnetic field or of an initial magnetization field with a flip conductor are not required, and a homogeneous guidance of the magnetic field of the permanent-magnetic elements through the resistor element can be achieved. The design of the sensor chip is simple to manufacture, requires fewer electrical connections, and control electronics of a simple design. Current consumption is lowered.
In principle, the linearization angle can assume any value between 0° and 90°. In accordance with one advantageous embodiment of the invention, the value of the linearization angle |α|=45°. The linearization angle α can be set to any desired value, and is only defined by the arrangement of the magnetization axis of the magnetization element relative to the axis of the flow of current through the AMR resistor element. By a setting of 45°, a perfect linearization of the AMR characteristic resistor curve is achieved relative to the AMR characteristic resistance curve, so that the value of the external magnetic field can be verified directly from the change in resistance, in particular as long as He<H0 applies. The evaluation electronics are thus simplified, and increased precision of measurement is achieved.
In principle, a single resistance element is sufficient for the measurement of a magnetic field vector component. In accordance with an advantageous development, each resistor device can comprise a series circuit of more than 10 AMR resistor elements which are preferably arranged along one or more straight and parallel rows on the chip substrate. The magnetization axis of the magnetization elements can be aligned parallel or perpendicular to the longitudinal extent of the resistor device. The arrangement often or more AMR resistor elements results in an increased value of the AMR resistance, so that a current consumption is reduced, thus lowering electrical losses. The resistor elements are preferably arranged in a straight row, and a plurality of parallel rows can be connected electrically in series in order to form a single resistor device. An increased packing density is achieved by this means. Each individual resistor element can preferably have a resistance value of between 5Ω and 50Ω. A series circuit thus yields a resistance value of at least 50Ω, which can typically adopt values of several 100Ω to a few kΩ. This means that a lower measurement current and hence a lower energy consumption by the apparatus can be achieved.
The magnetization axis of the magnetization element is preferably parallel, perpendicular or 45° to the longitudinal extent of the AMR resistor elements arranged in series. Accordingly, the contacting locations and the shape of the conductive strips are arranged and designed in such a way that the current flows at an angle α relative to the initial magnetization M0 of the resistor element. The magnetization elements arranged on the resistor device can here be aligned in the same way for all the resistor elements. This allows a single resistor or a resistor device arranged on a chip substrate in a compact and spacesaving manner to be provided.
In accordance with one advantageous development, a resistor element can be formed as a platelet and can have a symmetrical footprint, in particular a footprint of a regular polygon, a circle or an ellipse. By the platelet design, which results in a low thickness in comparison with the length and width of the resistor element is given, and by a symmetrical footprint, an arrangement of a large number of resistor devices is permitted on one chip substrate, where the effective area of the chip substrate can be exploited optimally. For example, a highly concentrated AMR resistor device having a high sensitivity can thus be arranged in a small space. By designing the resistor element with a symmetrical footprint, the direction of current flow is predetermined solely by the contact point and by the shape of the conductive strips contacting the resistor element. A symmetry of the resistor element relative to the contact points or the contact areas, and in relation to the magnetization vector of the magnetizing element has on the one hand the advantage that a homogeneous current distribution of the measurement current IS through the resistor element is achieved, while on the other hand a uniform initial magnetization M0 is given within the resistor element. In this way, the direction of the flow of current and the magnetic field permeating through the AMR resistor element, and hence 3 the linearization angle α, is defined exactly.
In accordance with one advantageous development, one permanent magnetization element can be assigned to each resistor element, where a magnetic flux of the magnetization element passes at least partly through the resistor element. A permanent magnetization element consists preferably of a ferromagnetic material such as iron, cobalt, nickel or ferrite, or an alloy of these, for example CoPtCr, or rare-earth compounds such as CoSm or NdFeB, which generate a static magnetic field with a north and a south pole. Magnetic field lines emerge from both polar ends, and are closed again by an external stray field. In the region of this stray field, the resistor element is arranged such that the stray field passes through the resistor element from north pole to south pole at a linearization angle α relative to the direction of current flow. In this embodiment, a single permanent magnetization element, i.e. an elementary magnet, is assigned to each resistor element. The pairing of a resistor element and a permanent magnetization element here represents the smallest possible unit of a resistor device incorporated in the magnetic field sensor apparatus.
In the embodiment mentioned above, it is particularly advantageous for the magnetization element to cover the full area of the resistor element, so that the stray field passes through the AMR resistor element in large areas. In this way, a good initial magnetization of the AMR element by the initial magnetization field H0 is achieved.
As an alternative to the embodiments mentioned above, it can be advantageous for two permanent magnetization elements aligned along a common magnetization axis to be assigned to the resistor element of the resistor device, where a magnetic flux from the north pole of one magnetization element to the south pole of the adjacent magnetization element passes at least partly through the resistor element. It is thus proposed in this embodiment that two or more magnetization elements are arranged in a row, where a magnetic flux from the respective north pole of one magnetization element to the south pole of the respective adjacent magnetization element results. The permanent magnetization element is arranged in this magnetization flux in such a way that the stray field between the two magnetization elements passes through it. There is no 1:1 assignment of a single magnetization element to a resistor element here, but rather each resistor element is arranged in the space in between a pair of magnetization elements. This means that magnetization elements and resistor elements do not overlap, but are arranged offset relative to one another.
In the aforementioned exemplary embodiment, it can furthermore be advantageous for the resistor element to be arranged between the end regions of the two magnetization elements, where the end regions facing towards them preferably enclose the resistor element with a complementary shape along at least a section of its periphery. It is thus advantageously proposed that the end region of each magnetization element has a peripheral line such that it encloses the periphery of the resistor element with a complementary shape, so that an undisturbed magnetic flux can pass from the north pole of the one magnetization element through the AMR resistor element to the adjacent south pole of the second magnetization element. Stray field losses are thus low, and a homogeneous initial magnetization field H0 can be achieved.
In accordance with a further advantageous embodiment of the invention, the layer thickness of the resistor element and of the magnetization element are at most 10 μm, preferably <1 μm, in particular <50 nm, where in particular the layer thickness of the magnetization element is not more than 10 times the layer thickness of the resistor element. As a result of the size of the layer thicknesses, which is at least smaller than 10 μm and preferably <1 μm, it is ensured that a compact chip structure for the AMR magnetic field sensor apparatus is achieved, with no need to employ macroscopic magnetic field coils or the like. The permanent magnet elements, like the AMR resistor strips, are located on the same or on different planes within a chip substrate, and can be integrated into a compact chip structure.
In accordance with a further advantageous embodiment of the invention, at least two and in particular several resistor devices are comprised in one apparatus, where each resistor device comprises a plurality of resistor elements arranged in straight and parallel rows and said rows of two resistor devices are arranged to mesh with one another interdigitally, where the linearization angles of the interdigitally arranged resistor devices are preferably displaced by 90° relative to one another, and where the resistor devices are in particular interconnected in the form of a half-bridge or a full bridge. In this embodiment, a magnetic field sensor apparatus is proposed comprising two or more, and preferably four, resistor devices, i.e. individual and separately contactable AMR measuring resistors that are arranged nested compactly together on the chip substrate. Each resistor device consists of an electrical series connection of several parallel resistor elements arranged in straight rows, and the rows of the resistor elements belonging to the resistor devices mesh interdigitally in rows of an adjacent resistor device in order to achieve a compact design. Since the rows of different resistor devices are arranged nested with one another, the process technology can achieve a similar behavior of the resistor devices connected to form a half-bridge or full bridge, so that an equivalent resistance behavior of the resistors interconnected in a measuring bridge circuit, i.e. resistor devices, is ensured.
In accordance with an advantageous development of the invention, it is possible for the magnetization elements to be arranged in a lower planar layer on the chip substrate, on top of that a layer comprising the AMR resistor elements, and on top of that a layer comprising the contacting strips. This design proposes that on the chip substrate, which for example also has an insulation layer applied, the magnetization elements consisting of a permanent-magnetic material such as CoPtCr are arranged first. Building on top of that, in a layer located above, the resistor elements are arranged in such a way that a 1:1 ratio or a 1:2 ratio is established between resistor elements and magnetization elements. As a general rule, the magnetization elements have a larger surface area than the resistor elements. After this, the contact strips are arranged between the AMR resistor elements and electrical contact is made with them in such a way that a current flowing through the rows of resistor elements has a linearization angle α relative to the initial magnetization M0 of the resistor elements. A design of this type has the advantage that the magnetization elements, which are designed in their surface area, are arranged on a lower plane, and the resistor elements with their smaller shape and the contact strips are arranged on higher planes of the chip substrate, so that the usual methods can be used for manufacturing the magnetic field sensor apparatus.
Starting from the aforementioned layer structure of the magnetic field sensor apparatus, it can be advantageous if an intermediate layer which is essentially magnetically neutral is arranged between the layer of resistor elements and the layer of the magnetization elements, where the intermediate layer consists of an electrical insulator which prevents short circuits between the resistor elements via the electrically conductive magnetization elements. This layout, which is made very thin in comparison with the magnetization elements and resistor elements, is preferably magnetically neutral, i.e. has a permeability μ=1 (paramagnetic), in order not to disturb the magnetic flux.
It is furthermore proposed that an insulating covering layer is applied on top of the layer comprising the contacting strips, so that the contacting strips are insulated from the chip surface by a covering layer. In this way, a layer structure is proposed for the chip substrate that describes a highly compact and easily manufactured magnetic field sensor apparatus.
In accordance with a further development of the aforementioned chip structure, it can be advantageous if a compensation conductor is arranged on one or more further planar layers, in particular on an upper planar layer, through which conductor a compensation current IC can flow in order to generate a compensation magnetic field perpendicular to the magnetization axis of the initial magnetization M0 and thus against the direction of the external magnetic field He to be measured. The compensation current IC of the compensation conductor can compensate for a component to be measured of an external magnetic field at the magnetically sensitive regions of the sensor. Thus a compensation current IC can for example flow through the compensation conductor, generating a magnetic field opposed to the direction of the magnetic field He to be measured, so that the external magnetic field He to be measured is compensated, or its effect attenuated, so that a high sensitivity, i.e. linearity between the characteristic resistance curve and the external magnetic field, can be achieved. It is possible with the compensation magnetic field to measure magnetic fields with different scale magnitudes, and to adjust or calibrate the measuring range of the magnetic field sensor apparatus. The magnitude of the compensation current IC thus provides information about the size of the external magnetic field. The compensation conductor is aligned in the direction of the initial magnetization M0 at the location of the resistor element, and the direction of the flow of current of the compensation current IC provides information about the direction of the external magnetic field He.
In a subsidiary aspect, a compass system is proposed in which one, two or three magnetic field sensor apparatuses are arranged at angles to one another, preferably 90° to one another, which can perform a one, two or threedimensional measurement of a magnetic field, where in particular in an electronic module an acquisition of the directional magnitude of the magnetic field, in particular of the Earth's magnetic field, can be performed with the aid of a magnetic field sensor arrangement presented above. Compass systems of this type are found in various applications and devices, for example navigation systems, Smartphones, tablet computers or other applications in which a direction of a magnetic field, in particular of the Earth's magnetic field, is to be displayed. A compass system of this type, fitted with the above-described magnetic field sensor apparatus, exploits its advantages and has in particular a reduced current consumption, longer service life and lower component costs.
Furthermore, in a subsidiary aspect, a current measuring system is proposed comprising at least one magnetic field sensor apparatus as discussed above, in order to determine a current magnitude of one or more conductors. A conductor through which current flows generates a magnetic field surrounding it, where it is possible by measurement of the strength of the magnetic field surrounding the conductor to determine the current that is flowing through the conductor without making contact. Current measuring systems of this type are widely used as transportable systems, but also in stationary form to monitor the flow of current inside machines or equipment, for example electricity distribution installations. Apart from the connections for the measurement current through the AMR resistor devices, and possibly current connections for the compensation conductor, the proposed passive magnetic field sensor apparatus does not comprise any other connections, and in particular no connections for a flip current conductor, so that contactless current measurement can be performed by a relatively simple electronic evaluation unit. The current measurement system can comprise a plurality of measuring apparatuses of this type to permit an extremely precise measurement of current.
Elements that are of the same type are given the same reference character in the figures.